Ign published an interview with some informations about the new gamemode that OOC hasn't covered yet. So I thought I'd give you a short summary of all the new information:

-Mercenaries has its own 15 person dev team that is actively working on the mode for over 11 months. The dev team also includes Paul Nygen who pitched the idea of the Dungeon Run mode.

-Mercenaries was originially planned as an 2v2 mode, but they then settled on the rogue-lite (I know the devs are calling it a rogue-like, but a rogue-like with permanent progression is per definition a rogue-lite) and rpg mechanics

- they refear to the runs as "levels" (they also did that other times they talked about it)

- equipment can come from "drops in levels" or from other sources

- the max level could be level 30 ("level 1 to, say, 30")

- the endgame is about enhancing your characters and equipment

- There will be an heroic option

- some abilities will be better against certain bosses

- a lot of endgame gameplay will be about figuering out how to counter a certain boss

- Jaina and the Lich King will be bosses

- You can see what the enemy will do in the pve parts. That's not the case for pvp

- As a result the pve part will be more about puzzles, and the pvp about bluffing and playing mind games (probably not the card Mindgames )

- they announced the mode at Blizzconline to show everyone that Hearthstone is doing more than just releasing expansions.

Bonus from an GamesPredetor Interview I stumbled across:

-The mode will have no mana system and the abilites are cooldown based instead

- example abilities could be a simple attack or a Dragon breath that either deals damage, or heals an ally.

- they think that the mode will be really deep

- fights will reward you with xp and "resources"

All of this sounds super exciting and I can't wait to learn out more.